After____was about ten minutes,the teacher gave the students the correct answer to the question.为什么不能填it,把after看做连词,it当做时间呢

After what was...
after 的后面必须接某时间,某事件,某动作,而it was about ten minutes是一个判断或说明,因此,after it was ...在意义上是讲不通的.
而在这个句子中,what = some time that,可以变成Ater some time that was about ten minutes,...